The long-term aim is to develop (eventually in collaboration with others) a rational and more effective aid for smoking cessation. In this regard, nicotine blockade therapy represents a promising new approach. The challenge is to develop a drug which blocks nicotine for several weeks after only one administration in animals, but appears to block autonomic ganglia for only a few hours. The present objective is to investigate the mechanism(s) underlying chlorisondamine's persistent blockade. There are several possibilities, some of which would preclude the therapeutic use of chlorisondamine; chlorisondamine may become trapped in the brain by diffusional barriers; it may bind irreversibly to nicotinic cholinoceptors (nAChRs); it may damage neurons, possibly entering via nAChR- associated ionophores; and it may inhibit the expression of nAChRs by some other mechanism. The following specific hypotheses will be tested. (1) After in vivo administration, chlorisondamine remains in the CNS for long periods comparable to its duration of blockade. (20 The drug preferentially accumulates in certain brain areas in vivo. (3) Chlorisondamine is bound to or taken up by a finite population of sites in vivo. (4) 3H-chlorisondamine could serve as an irreversible and selective ligand for nAChRs in the brian (5) The drug is neurotoxic, and this action is associated with nAChRs (6) Chlorisondamine reduces the density of nAChRs in brain without destroying neurons. All experiments will be performed in rats. After in vivo administration, the neuroanatomical and cellular distribution of radiolabeled chlorisondamine will be investigated in the CNS by autoradiography. The pharmacology and anatomy of putative chlorisondamine binding sites in brain will be investigated in vivo and in vitro, and compared to that of nAChRs. Possible neurotoxic effects will be assessed histologically. The effects of chlorisondamine on nAChr density will be investigated by 3H-nicotine autoradiography.